Cold Rolled Steel Sheet Having Superior Formability, Process for Producing the Same

ABSTRACT

Disclosed herein is a Ti-based IF steel in which fine precipitates, such as CuS precipitates, having a size of 0.2 μm or less are distributed. The distribution of fine precipitates in the Ti-based IF steel enhances the yield strength and lowers the in-plane anisotropy index. The nanometer-sized precipitates allow the formation of minute crystal grains. As a result, dissolved carbon is present in a larger amount in the crystal grain boundaries than within the crystal grains, which is advantageous in terms of room-temperature non-aging properties and bake hardenability.

TECHNICAL FIELD

The present invention relates to titanium (Ti) based interstitial free(IF) cold rolled steel sheets that are used as materials forautomobiles, household electronic appliances, etc. More specifically,the present invention relates to highly formable Ti based IF cold rolledsteel sheets whose yield strength is enhanced due to the distribution offine precipitates, and a process for producing the Ti-based IF coldrolled steel sheets.

BACKGROUND ART

In general, cold rolled steel sheets for use in automobiles andhousehold electronic appliances are required to have excellentroom-temperature aging resistance and bake hardenability, together withhigh strength and superior formability.

Aging is a strain aging phenomenon that arises from hardening caused bydissolved elements, such as C and N, fixed to dislocations. Since agingcauses defect, called “stretcher strain”, it is important to secureexcellent room-temperature aging resistance.

Bake hardenability means increase in strength due to the presence ofdissolved carbon after press formation, followed by painting and drying,by leaving a slight small amount of carbon in a solid solution state.Steel sheets with excellent bake hardenability can overcome thedifficulties of press formability resulting from high strength.

Room-temperature aging resistance and bake hardenability can be impartedto aluminum (Al)-killed steels by batch annealing of the Al-killedsteels. However, extended time of the batch annealing causes lowproductivity of the Al-killed steels and severe variation in steelmaterials at different sites. In addition, Al-killed steels have a bakehardening (BH) value (a difference in yield strength before and afterpainting) of 10-20 MPa, which demonstrates that an increase in yieldstrength is low.

Under such circumstances, interstitial free (IF) steels with excellentroom-temperature aging resistance and bake hardenability have beendeveloped by adding carbide and nitride-forming elements, such as Ti andNb, followed by continuous annealing.

For example, Japanese Unexamined Patent Publication No. Sho 57-041349describes an enhancement in the strength of a Ti-based IF steel byadding 0.4-0.8% of manganese (Mn) and 0.04-0.12% of phosphorus (P). Invery low carbon IF steels, however, P causes the problem of secondaryworking embrittlement due to segregation in grain boundaries.

Japanese Unexamined Patent Publication No. Hei 5-078784 describes anenhancement in strength by the addition of Mn as a solid solutionstrengthening element in an amount exceeding 0.9% and not exceeding3.0%.

Korean Patent Laid-open No. 2003-0052248 describes an improvement insecondary working embrittlement resistance as well as strength andworkability by the addition of 0.5-2.0% of Mn instead of P, togetherwith aluminum (Al) and boron (B).

Japanese Unexamined Patent Publication No. Hei 10-158783 describes anenhancement in strength by reducing the content of P and using Mn and Sias solid solution strengthening elements. According to this publication,Mn is used in an amount of up to 0.5%, Al as a deoxidizing agent is usedin an amount of 0.1%, and nitrogen (N) as an impurity is limited to0.01% or less. If the Mn content is increased, the platingcharacteristics are worsened.

Japanese Unexamined Patent Publication No. Hei 6-057336 discloses anenhancement in the strength of an IF steel by adding 0.5-2.5% of copper(Cu) to form E-Cu precipitates. High strength of the IF steel isachieved due to the presence of the ε-Cu precipitates, but theworkability of the IF steel is worsened.

Japanese Unexamined Patent Publication Nos. Hei 9-227951 and Hei10-265900 suggest technologies associated with improvement inworkability or surface defects due to carbides by the use of Cu as anucleus for precipitation of the carbides. According to the formerpublication, 0.005-0.1% of Cu is added to precipitate CuS during temperrolling of an IF steel, and the CuS precipitates are used as nuclei toform Cu—Ti—C—S precipitates during hot rolling. In addition, the formerpublication states that the number of nuclei forming a {111} planeparallel to the surface of a plate increases in the vicinity of theCu—Ti—C—S precipitates during recrystallization, which contributes to animprovement in workability. According to the latter publication,0.01-0.05% of Cu is added to an IF steel to obtain CuS precipitates andthen the CuS precipitates are used as nuclei for precipitation ofcarbides to reduce the amount of dissolved carbon (C), leading to animprovement in surface defects. According to the prior art, since coarseCuS precipitates are used during production of cold rolled steel sheets,carbides remain in the final products. Further, since emulsion-formingelements, such as Ti and Zr, are added in an amount greater than theamount of sulfur (S) in an atomic weight ratio, a main portion of thesulfur (S) reacts with Ti or Zr rather than Cu.

On the other hand, Japanese Unexamined Patent Publication Nos. Hei6-240365 and Hei 7-216340 describe the addition of a combination of Cuand P to improve the corrosion resistance of baking hardening type IFsteels. According to these publications, Cu is added in an amount of0.05-1.0% to ensure improved corrosion resistance. However, inactuality, Cu is added in an excessively large amount of 0.2% or more.

Japanese Unexamined Patent Publication Nos. Hei 10-280048 and Hei10-287954 suggest the dissolution of carbosulfide (Ti—C—S based) in acarbide at the time of reheating and annealing to obtain a solidsolution in crystal grain boundaries, thereby achieving a bake hardening(BH) value (a difference in yield strength before and after baking) of30 MPa or more.

According to the aforementioned publications, strength is enhanced bystrengthening solid solution or using ε-Cu precipitates. Cu is used toform ε-Cu precipitates and improve corrosion resistance. In addition, Cuis used as a nucleus for precipitation of carbides. No mention is madein these publications about an increase in high yield ratio (i.e. yieldstrength/tensile strength) and a reduction in in-plane anisotropy index.If the tensile strength-to-yield strength ratio (i.e. yield ratio) of anIF steel sheet is high, the thickness of the IF steel sheet can bereduced, which is effective in weight reduction. In addition, if thein-plane anisotropy index of an IF steel sheet is low, fewer wrinklesand ears occur during processing and after processing, respectively.

DISCLOSURE Technical Problem

It is one object of certain embodiments of the present invention toprovide Ti based IF cold rolled steel sheets that are capable ofachieving a high yield ratio and a low in-plane anisotropy index.

It is another object of certain embodiments of the present invention toprovide a process for producing the IF cold rolled steel sheets.

Technical Solution

According to the present invention, there is provided a cold rolledsteel sheet which has a composition comprising 0.01% or less of C,0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less ofN, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight,and the balance of Fe and other unavoidable impurities, wherein thecomposition satisfies the following relationships:1≦(Cu/63.5)/(S*/32)≦30 and S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and thesteel sheet comprises CuS precipitates having an average size of 0.2 μmor less.

According to the present invention, there is provided a cold rolledsteel sheet which has a composition comprising 0.01% or less of C,0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005-0.08% of S, 0.1% or less of Al,0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15%of Ti, by weight, and the balance of Fe and other unavoidableimpurities, wherein the composition satisfies the followingrelationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30 andS=S*−0.8×(Ti−0.8×(48/14)×N)×(32/48), and the steel sheet comprises(Mn,Cu)S precipitates having an average size of 0.2 μm or less.

According to the present invention, there is provided a cold rolledsteel sheet which has a composition comprising 0.01% or less of C,0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004-0.02% of N,0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, andthe balance of Fe and other unavoidable impurities, wherein thecomposition satisfies the following relationships:1≦(Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10,S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) andN*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises CuSand AlN precipitates having an average size of 0.2 μm or less.

According to the present invention, there is provided a cold rolledsteel sheet which has a composition comprising 0.01% or less of C,0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005-0.08% of S, 0.1% or less of Al,0.004-0.02% of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% ofTi, by weight, and the balance of Fe and other unavoidable impurities,wherein the composition satisfies the following relationships:1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10,S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) andN*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises(Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.

According to the present invention, there is provided a cold rolledsteel sheet which has a composition comprising 0.01% or less of C, 0.08%or less of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less ofP, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least one kind selectedfrom 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, by weight,and the balance of Fe and other unavoidable impurities, wherein thecomposition satisfies the following relationships:1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10, where the N contentis 0.004% or more, S=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) andN*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), the steel sheet comprises at leastone kind selected from (Mn,Cu)S and AlN precipitates having an averagesize of 0.2 μm or less.

When the cold rolled steel sheets of the present invention satisfy thefollowing relationships between the C, Ti, N and S contents:0.8≦(Ti*/48)/(C/12)≦5.0 and Ti=Ti−0.8×((48/14)×N+(48/32)×S), they showroom-temperature non-aging properties. In addition, when solute carbon(Cs) [Cs=(C−Ti*×12/48)×10000 in which Ti*=Ti−0.8×((48/14)×N+(48/32)×S),provided that when Ti* is less than 0, Ti* is defined as 0], which isdetermined by the C and Ti contents, is from 5 to 30, the cold rolledsteel sheets of the present invention show bake hardenability.

Depending on the design of the compositions, the cold rolled steelsheets of the present invention have characteristics of soft cold rolledsteel sheets of the order of 280 MPa and high-strength cold rolled steelsheets of the order of 340 MPa or more.

When the content of P in the compositions of the present invention is0.015% or less, soft cold rolled steel sheets of the order of 280 MPaare produced. When the soft cold rolled steel sheets further contain atleast one solid solution strengthening element selected from Si and Cr,or the P content is in the range of 0.015-0.2%, a high strength of 340MPa or more is attained. The P content in the high-strength steelscontaining P alone is preferably in the range of 0.03% to 0.2%. The Sicontent in the high-strength steels is preferably in the range of 0.1 to0.8%. The Cr content in the high-strength steels is preferably in therange of 0.2 to 1.2. In the case where the cold rolled steel sheets ofthe present invention contain at least one element selected from Si andCr, the P content may be freely designed in an amount of 0.2% or less.

For better workability, the cold rolled steel sheets of the presentinvention may further contain 0.01-0.2 wt % of Mo.

According to the present invention, there is provided a process forproducing the cold rolled steel sheets, the process comprising reheatinga slab satisfying one of the compositions to a temperature of 1,100° C.or higher, hot rolling the reheated slab at a finish rolling temperatureof the Ar₃ transformation point or higher to provide a hot rolled steelsheet, cooling the hot rolled steel sheet at a rate of 300° C./min.,winding the cooled steel sheet at 700° C. or lower, cold rolling thewound steel sheet, and continuously annealing the cold rolled steelsheet.

BEST MODE

The present invention will be described in detail below.

Fine precipitates having a size of 0.2 μm or less are distributed in thecold rolled steel sheets of the present invention. Examples of suchprecipitates include MnS precipitates, CuS precipitates, and compositeprecipitates of MnS and CuS. These precipitates are referred to simplyas “(Mn,Cu)S”.

The present inventors have found that when fine precipitates aredistributed in Ti-based IF steels, the yield strength of the IF steelsis enhanced and the in plane anisotropy index of the IF steels islowered, thus leading to an improvement in workability. The presentinvention has been achieved based on this finding. The precipitates usedin the present invention have drawn little attention in conventional IFsteels. Particularly, the precipitates have not been actively used fromthe viewpoint of yield strength and in-plane anisotropy index.

Regulation of the components in the Ti-based IF steels is required toobtain (Mn,Cu)S precipitates and/or AlN precipitates. If the IF steelscontain Ti, Zr and other elements, S and N preferentially react with Tiand Zr. Since the cold rolled steel sheets of the present invention areTi added IF steels, Ti reacts with C, N and S. Accordingly, it isnecessary to regulate the components so that S and N are precipitatedinto (Mn,Cu)S and AlN forms, respectively.

The fine precipitates thus obtained allow the formation of minutecrystal grains. Minuteness in the size of crystal grains relativelyincreases the proportion of crystal grain boundaries. Accordingly, thedissolved carbon is present in a larger amount in the crystal grainboundaries than within the crystal grains, thus achieving excellentroom-temperature non-aging properties. Since the dissolved carbonpresent within the crystal grains can more freely migrate, it binds tomovable dislocations, thus affecting the room-temperature agingproperties. In contrast, the dissolved carbon segregated in stablepositions, such as in the crystal grain boundaries and in the vicinityof the precipitates, is activated at a high temperature, for example, atemperature for painting/baking treatment, thus affecting the bakehardenability.

The fine precipitates distributed in the steel sheets of the presentinvention have a positive influence on the increase of yield strengtharising from precipitation enhancement, improvement instrength-ductility balance, in-plane anisotropy index, and plasticityanisotropy. To this end, the fine (Mn,Cu)S precipitates and AlNprecipitates must be uniformly distributed. According to the cold rolledsteel sheets of the present invention, contents of components affectingthe precipitation, composition between the components, productionconditions, and particularly cooling rate after hot rolling, have agreat influence on the distribution of the fine precipitates.

The constituent components of the cold rolled steel sheets according tothe present invention will be explained.

The content of carbon (C) is preferably limited to 0.01% or less.

Carbon (C) affects the room-temperature aging resistance and bakehardenability of the cold rolled steel sheets. When the carbon contentexceeds 0.01%, the addition of the expensive agents Ti is required toremove the remaining carbon, which is economically disadvantageous andis undesirable in terms of formability. When it is intended to achieveroom-temperature aging resistance only, it is preferred to maintain thecarbon content at a low level, which enables the reduction of the amountof the expensive agents Ti added. When it is intended to ensure desiredbake hardenability, the carbon is preferably added in an amount of0.001% or more, and more preferably 0.005% to 0.01%. When the carboncontent is less than 0.005%, room-temperature aging resistance can beensured without increasing the amounts of Ti.

The content of copper (Cu) is preferably in the range of 0.01-0.2%.

Copper serves to form fine CuS precipitates, which make the crystalgrains fine. Copper lowers the in-plane anisotropy index of the coldrolled steel sheets and enhances the yield strength of the cold rolledsteel sheets by precipitation promotion. In order to form fineprecipitates, the Cu content must be 0.01% or more. When the Cu contentis more than 0.2%, coarse precipitates are obtained. The Cu content ismore preferably in the range of 0.03 to 0.2%.

The content of manganese (Mn) is preferably in the range of 0.01-0.3%.

Manganese serves to precipitate sulfur in a solid solution state in thesteels as MnS precipitates, thereby preventing occurrence of hotshortness caused by the dissolved sulfur, or is known as a solidsolution strengthening element. From such a technical standpoint,manganese is generally added in a large amount. The present inventorshave found that when the manganese content is reduced and the sulfurcontent is optimized, very fine MnS precipitates are obtained. Based onthis finding, the manganese content is limited to 0.3% or less. In orderto ensure this characteristic, the manganese content must be 0.01% ormore. When the manganese content is less than 0.01%, i.e. the sulfurcontent remaining in a solid solution state is high, hot shortness mayoccur. When the manganese content is greater than 0.3%, coarse MnSprecipitates are formed, thus making it difficult to achieve desiredstrength. A more preferable Mn content is within the range of 0.01 to0.12%.

The content of sulfur (S) is preferably limited to 0.08% or less.

Sulfur (S) reacts with Cu and/or Mn to form CuS and MnS precipitates,respectively. When the sulfur content is greater than 0.08%, theproportion of dissolved sulfur is increased. This increase of dissolvedsulfur greatly deteriorates the ductility and formability of the steelsheets and increases the risk of hot shortness. In order to obtain asmany CuS and/or MnS precipitates as possible, a sulfur content of 0.005%or more is preferred.

The content of aluminum (Al) is preferably limited to 0.1% or less.

Aluminum reacts with nitrogen (N) to form fine AlN precipitates, therebycompletely preventing aging by dissolved nitrogen. When the nitrogencontent is 0.004% or more, AlN precipitates are sufficiently formed. Thedistribution of the fine AlN precipitates in the steel sheets allows theformation of minute crystal grains and enhances the yield strength ofthe steel sheets by precipitation enhancement. A more preferable Alcontent is in the range of 0.01 to 0.1%.

The content of nitrogen (N) is preferably limited to 0.02% or less.

When it is intended to use AlN precipitates, nitrogen is added in anamount of up to 0.02%. Otherwise, the nitrogen content is controlled to0.004% or less. When the nitrogen content is less than 0.004%, thenumber of the AlN precipitates is small, and therefore, the minutenesseffects of crystal grains and the precipitation enhancement effects arenegligible. In contrast, when the nitrogen content is greater than0.02%, it is difficult to guarantee aging properties by use of dissolvednitrogen.

The content of phosphorus (P) is preferably limited to 0.2% or less.

Phosphorus is an element that has excellent solid solution strengtheningeffects while allowing a slight reduction in r-value. Phosphorusguarantees high strength of the steel sheets of the present invention inwhich the precipitates are controlled. It is desirable that thephosphorus content in steels requiring a strength of the order of 280MPa be defined to 0.015% or less. It is desirable that the phosphoruscontent in high-strength steels of the order of 340 MPa be limited to arange exceeding 0.015% and not exceeding 0.2%. A phosphorus contentexceeding 0.2% can lead to a reduction in ductility of the steel sheets.Accordingly, the phosphorus content is preferably limited to a maximumof 0.2%. When Si and Cr are added in the present invention, thephosphorus content can be appropriately controlled to be 0.2% or less toachieve the desired strength.

The content of boron (B) is preferably in the range of 0.0001 to 0.002%.

Boron is added to prevent occurrence of secondary working embrittlement.To this end, a preferable boron content is 0.0001% or more. When theboron content exceeds 0.002%, the deep drawability of the steel sheetsmay be markedly deteriorated.

The content of titanium (Ti) is preferably in the range of 0.005 to−0.15%.

Titanium is added for the purpose of ensuring the non-aging propertiesand improving the formability of the steel sheets. Ti, which is a potentcarbide-forming element, is added to steels to form TiC precipitates inthe steels. The TiC precipitates allow the precipitation of dissolvedcarbon to ensure non-aging properties. When the content of Ti added isless than 0.005%, the TiC precipitates are obtained in very smallamounts. Accordingly, the steel sheets are not well textured and thusthere is little improvement in the deep drawability of the steel sheets.In contrast, when the titanium is added in an amount exceeding 0.15%,very large TiC precipitates are formed. Accordingly, minuteness effectsof crystal grains are reduced, resulting in high in-plane anisotropyindex, reduction of yield strength and marked worsening of platingcharacteristics.

To obtain (Mn,Cu)S and AlN precipitates, the Mn, Cu, S, Ti, Al, N and Ccontents are adjusted within the ranges defined by the followingrelationships. The respective components indicated in the followingrelationships are expressed as percentages by weight.

1≦(Cu/63.5)/(S*/32)≦30  (1)

S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48)  (2)

In Relationship 1, S*, which is determined by Relationship 2, representsthe content of sulfur that does not react with Ti and thereafter reactswith Cu. To obtain fine CuS precipitates, it is preferred that the valueof (Cu/63.5)/(S*/32) be equal to or greater than 1. If the value of(Cu/63.5)/(S*/32) is greater than 30, coarse CuS precipitates aredistributed, which is undesirable. To stably obtain CuS precipitateshaving a size of 0.2 um or less, the value of (Cu/63.5)/(S*/32) ispreferably in the range of 1 to 20, more preferably 1 to 9, and mostpreferably 1 to 6.

1≦(Mn/55+Cu/63.5)/(S*/32)≦30  (3)

Relationship 3 is associated with the formation of (Mn,Cu)Sprecipitates, and is obtained by adding a Mn content to Relationship 1.To obtain effective (Mn,Cu)S precipitates, the value of(Mn/55+Cu/63.5)/(S*/32) must be 1 or greater. When the value ofRelationship 3 is greater than 30, coarse (Mn,Cu)S precipitates areobtained. To stably obtain (Mn,Cu)S precipitates having a size of 0.2 μmor less, a more preferable value of (Cu/63.5)/(S*/32) is preferably inthe range of 1 to 20, more preferably 1 to 9, and most preferably 1 to6. When Mn and Cu are added together, the sum of Mn and Cu is morepreferably 0.05-0.4%. The reason for this limitation to the sum of Mnand Cu is to obtain fine (Mn,Cu)S precipitates.

1≦(Al/27)/(N*/14)≦10  (4)

N=N−0.8×(Ti−0.8×(48/32)×S))×(14/48)  (5)

Relationship 4 is associated with the formation of fine (Mn,Cu)Sprecipitates. In Relationship 4, N*, which is determined by Relationship5, represents the content of nitrogen that does not react with Ti andthereafter reacts with Al. To obtain fine AlN precipitates, it ispreferred that the value of (Al/27)/(N*/14) be in the range of 1-10. Toobtain effective AlN precipitates, the value of (Al/27)/(N*/14) must be1 or greater. If the value of (Al/27)/(N*/14) is greater than 10, coarseAlN precipitates are obtained and thus poor workability and low yieldstrength are caused. It is preferred that the value of (Al/27)/(N*/14)be in the range of 1 to 6.

The components of the cold rolled steel sheets according to the presentinvention may be combined in various ways according to the kind ofprecipitates to be obtained. For example, the present invention providesa cold rolled steel sheet which has a composition comprising 0.01% orless of C, 0.08% or less of S, 0.1% or less of Al, 0.004% or less of N,0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least onekind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N,by weight, and the balance of Fe and other unavoidable impuritieswherein the composition satisfies the following relationships:1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10 (with the provisothat the N content is 0.004% or more), S*=S−0.8×(Ti0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and thesteel sheet comprises at least one kind selected from MnS, CuS, MnS andAlN precipitates having an average size of 0.2 μm or less. That is, oneor more kinds selected from the group consisting of 0.01-0.2% of Cu,0.01-0.3% of Mn and 0.004-0.2% of N lead to various combinations of(Mn,Cu)S and AlN precipitates having a size not greater than 0.2 μm.

In the steel sheets of the present invention, carbon is precipitatedinto NbC and TiC forms. Accordingly, the room-temperature agingresistance and bake hardenability of the steel sheets are affecteddepending on the conditions of dissolved carbon under which NbC and TiCprecipitates are not obtained. Taking into account these requirements,it is most preferred that the Ti and C contents satisfy the followingrelationships.

0.8≦(Ti*/48)/(C/12)≦5.0  (6)

Ti*=Ti−0.8×((48/14)×N+(48/32)×S)  (7)

Relationship 6 is associated with the formation of TiC precipitates toremove the carbon in a solid solution state, thereby achievingroom-temperature non-aging properties. In Relationship 6, Ti*, which isdetermined by Relationship 7, represents the content of titanium thatreacts with N and S and thereafter reacts with C.

When the value of (Ti*/48)/(C/12) is less than 0.8, it is difficult toensure room-temperature non-aging properties. In contrast, when thevalue of (Ti*/48)/(C/12) is greater than 5, the amounts of Ti remainingin a solid solution state in the steels are large, which deterioratesthe ductility of the steels. When it is intended to achieveroom-temperature non-aging properties without securing bakehardenability, it is preferred to limit the carbon content to 0.005% orless. Although the carbon content is more than 0.005%, room-temperaturenon-aging properties can be achieved when Relationship 6 is satisfiedbut the amounts of TiC precipitates are increased, thus deterioratingthe workability of the steel sheets.

Cs=(C−Ti*×12/48)×10000  (8)

(provided that when Ti* is less than 0, Ti* is defined as 0.)

Relationship 8 is associated with the achievement of bake hardenability.Cs, which is expressed in ppm by Relationship 8, represents the contentof dissolved carbon that is not precipitated into TiC forms. In order toachieve a high bake hardening value, the Cs value must be 5 ppm or more.If the Cs value exceeds 30 ppm, the content of dissolved carbon isincreased, making it difficult to attain room-temperature non-agingproperties.

It is advantageous that the fine precipitates are uniformly distributedin the compositions of the present invention. It is preferable that theprecipitates have an average size of 0.2 μm or less. According to astudy conducted by the present inventors, when the precipitates have anaverage size greater than 0.2 μm, the steel sheets have poor strengthand low in-plane anisotropy index. Further, large amounts ofprecipitates having a size of 0.2 μm or less are distributed in thecompositions of the present invention. While the number of thedistributed precipitates is not particularly limited, it is moreadvantageous with higher number of the precipitates. The number of thedistributed precipitates is preferably 1×10⁵/mm² or more, morepreferably 1×10⁶/mm² or more, and most preferably 1×10⁷/mm² or more. Theplasticity-anisotropy index is increased and the in-plane anisotropyindex is lowered with increasing number of the precipitates, and as aresult, the workability is greatly improved. It is commonly known thatthere is a limitation in increasing the workability because the in-planeanisotropy index is increased with increasing plasticity-anisotropyindex. It is worth noting that as the number of the precipitatesdistributed in the steel sheets of the present invention increases, theplasticity-anisotropy index of the steel sheets is increased and thein-plane anisotropy index of the steel sheets is lowered. The steelsheets of the present invention in which the fine precipitates areformed satisfy a yield ratio (yield strength/tensile strength) of 0.58or higher.

When the steel sheets of the present invention are applied tohigh-strength steel sheets, they may further contain at least one solidsolution strengthening element selected from P, Si and Cr. The additioneffects of P have been previously described, and thus their explanationis omitted.

The content of silicon (Si) is preferably in the range of 0.1 to 0.8%.

Si is an element that has solid solution strengthening effects and showsa slight reduction in elongation. Si guarantees high strength of thesteel sheets of the present invention in which the precipitates arecontrolled. Only when the Si content is 0.1% or more, high strength canbe ensured. However, when the Si content is more than 0.8%, theductility of the steel sheets is deteriorated.

The content of chromium (Cr) is preferably in the range of 0.2 to 1.2%.

Cr is an element that has solid solution strengthening effects, lowersthe secondary working embrittlement temperature, and lowers the agingindex due to the formation of Cr carbides. Cr guarantees high strengthof the steel sheets of the present invention in which the precipitatesare controlled and serves to lower the in-plane anisotropy index of thesteel sheets. Only when the Cr content is 0.2% or more, high strengthcan be ensured. However, when the Cr content exceeds 1.2%, the ductilityof the steel sheets is deteriorated.

The cold rolled steel sheets of the present invention may furthercontain molybdenum (Mo).

The content of molybdenum (Mo) in the cold rolled steel sheets of thepresent invention is preferably in the range of 0.01 to 0.2%.

Mo is added as an element that increases the plasticity-anisotropy indexof the steel sheets. Only when the molybdenum content is not lower than0.01%, the plasticity-anisotropy index of the steel sheets is increased.However, when the molybdenum content exceeds 0.2%, theplasticity-anisotropy index is not further increased and there is adanger of hot shortness.

Production of Cold Rolled Steel Sheets

Hereinafter, a process for producing the cold rolled steel sheets of thepresent invention will be explained with reference to the preferredembodiments that follow. Various modifications of the embodiments of thepresent invention can be made, and such modifications are within thescope of the present invention.

The process of the present invention is characterized in that a steelsatisfying one of the steel compositions defined above is processedthrough hot rolling and cold rolling to form precipitates having anaverage size of 0.2 μm or less in a cold rolled sheet. The average sizeof the precipitates in the cold rolled plate is affected by the designof the steel composition and the processing conditions, such asreheating temperature and winding temperature. Particularly, coolingrate after hot rolling has a direct influence on the average size of theprecipitates.

Hot Rolling Conditions

In the present invention, a steel satisfying one of the compositionsdefined above is reheated, and is then subjected to hot rolling. Thereheating temperature is preferably 1,100° C. or higher. When the steelis reheated to a temperature lower than 1,100° C., coarse precipitatesformed during continuous casting are not completely dissolved andremain. The coarse precipitates still remain even after hot rolling.

It is preferred that the hot rolling is performed at a finish rollingtemperature not lower than the Ar₃ transformation point. When the finishrolling temperature is lower than the Ar₃ transformation point, rolledgrains are created, which deteriorates the workability and causes poorstrength.

The cooling is preferably performed at a rate of 300° C./min or higherbefore winding and after hot rolling. Although the composition of thecomponents is controlled to obtain fine precipitates, the precipitatesmay have an average size greater than 0.2 μm at a cooling rate of lessthan 300° C./min. That is, as the cooling rate is increased, many nucleiare created and thus the size of the precipitates becomes finer andfiner. Since the size of the precipitates' is decreased with increasingcooling rate, it is not necessary to define the upper limit of thecooling rate. When the cooling rate is higher than 1,000° C./min.,however, a significant improvement in the size reduction effects of theprecipitates is not further shown. Therefore, the cooling rate ispreferably in the range of 300-1000° C./min.

Winding Conditions

After the hot rolling, winding is performed at a temperature not higherthan 700° C. When the winding temperature is higher than 700° C., theprecipitates are grown too coarsely, thus making it difficult to ensurehigh strength.

Cold Rolling Conditions

The steel is cold rolled at a reduction rate of 50-90%. Since a coldreduction rate lower than 50% leads to creation of a small amount ofnuclei upon annealing recrystallization, the crystal grains are grownexcessively upon annealing, thereby coarsening of the crystal grainsrecrystallized through annealing, which results in reduction of thestrength and formability. A cold reduction rate higher than 90% leads toenhanced formability, while creating an excessively large amount ofnuclei, so that the crystal grains recrystallized through annealingbecome too fine, thus deteriorating the ductility of the steel.

Continuous Annealing

Continuous annealing temperature plays an important role in determiningthe mechanical properties of the final product. According to the presentinvention, the continuous annealing is preferably performed at atemperature of 700 to 900° C. When the continuous annealing is performedat a temperature lower than 700° C., the recrystallization is notcompleted and thus a desired ductility cannot be ensured. In contrast,when the continuous annealing is performed at a temperature higher than900° C., the recrystallized grains become coarse and thus the strengthof the steel is deteriorated. The continuous annealing is maintaineduntil the steel is completely recrystallized. The recrystallization ofthe steel can be completed for about 10 seconds or more. The continuousannealing is preferably performed for 10 seconds to 30 minutes.

[Mode for Invention]

The present invention will now be described in more detail withreference to the following examples.

The mechanical properties of steel sheets produced in the followingexamples were evaluated according to the ASTM E-8 standard test methods.Specifically, each of the steel sheets was machined to obtain standardsamples. The yield strength, tensile strength, elongation,plasticity-anisotropy index (r_(m) value) and in-plane anisotropy index(Δr value), and the aging index were measured using a tensile strengthtester (available from INSTRON Company, Model 6025). Theplasticity-anisotropy index r_(m) and in plane anisotropy index (Δrvalue) were calculated by the following equations: r_(m)=(r₀+2r₄₅+r₉₀)/4and Δr=(r₀−2r₄₅+r₉₀)/2, respectively.

The aging index of the steel sheets is defined as a yield pointelongation measured by annealing each of the samples, followed by 1.0%skin pass rolling and thermally processing at 100° C. for 2 hours. Thebake hardening (BH) value of the standard samples was measured by thefollowing procedure. After a 2% strain was applied to each of thesamples, the strained sample was annealed at 170° C. for 20 minutes. Theyield strength of the annealed sample was measured. The BH value wascalculated by subtracting the yield strength measured before annealingfrom the yield strength value measured after annealing.

EXAMPLE 1

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 1 Sample Chemical Components (wt %) No. C Cu S Al N P B Ti OthersA11 0.0008 0.17 0.026 0.027 0.0005 0.05 0.0004 0.039 Si: 0.02 A12 0.00150.09 0.037 0.042 0.0032 0.082 0.0007 0.059 Si: 0.15 A13 0.0028 0.120.047 0.023 0.0026 0.117 0.0012 0.075 Si: 0.25 A14 0.0015 0.08 0.0360.035 0.0014 0.083 0.0007 0.058 Si: 0.17 Mo: 0.07 A15 0.0017 0.11 0.050.034 0.0016 0.082 0.0009 0.072 Si: 0.18 Cr: 0.17 A16 0.0022 0.11 0.010.038 0.0015 0.059 0 0 A17 0.0046 0 0.011 0.029 0.0027 0.125 0.0008 0.16

TABLE 2 Average size of Number of (Mn/55 + CuS CuS Sample Cu/63.5)/(Ti^(★)/48)/ precipitates precipitates No. S^(★) (S^(★)/32) (C/12) (μm)(mm⁻²) A11 0.0059 14.443 2.01 0.06 3.2 × 10⁶ A12 0.0102 4.4402 0.97 0.064.1 × 10⁶ A13 0.0108 5.5975 1.02 0.06 4.5 × 10⁶ A14 0.0071 5.6665 1.830.05 5.1 × 10⁶ A15 0.0139 3.9764 1.12 0.05 4.3 × 10⁶ A16 0.0122 4.5458 00.08 4.5 × 10⁶ A17 0 0 7.58 0.08 6.7 × 10⁴ S^(★) = S − 0.8 × (Ti − 0.8 ×(48/14) × N) × (32/48), Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 3 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr AI (%) (DBTT-° C.) Remarks A11 219 348 46 2.22 0.34 0 −70 ISA12 260 398 40 1.93 0.32 0 −60 IS A13 325 451 37 1.85 0.36 0 −50 IS A14321 457 34 1.82 0.31 0 −50 IS A15 337 455 35 1.79 0.31 0 −60 IS A16 232348 43 1.12 0.29 0.62 −70 CS A17 275 448 28 1.82 0.48 0 −50 CS * Note:YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 2

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 4 Sample Chemical Components (wt % ) No. C Mn Cu S Al N P B TiOthers A21 0.0007 0.11 0.09 0.02 0.035 0.0008 0.043 0.0007 0.029 Si:0.08 A22 0.0012 0.08 0.12 0.032 0.039 0.0021 0.08 0.0009 0.049 Si: 0.17A23 0.0028 0.11 0.16 0.041 0.025 0.0019 0.11 0.0005 0.064 Si: 0.3 A240.0013 0.09 0.11 0.035 0.043 0.0023 0.082 0.0011 0.057 Si: 0.26 Mo: 0.1A25 0.0015 0.1 0.09 0.05 0.025 0.001 0.075 0.0012 0.069 Si: 0.32 Cr:0.21 A26 0.0035 0.45 0.14 0.009 0.033 0.0024 0.048 0.005 0 A27 0.00310.13 0.03 0.012 0.038 0.0021 0.118 0 0.15 Si: 0.33

TABLE 5 Average size of Number of (Mn/55 + (Mn, Cu)S (Mn, Cu)S SampleCu/63.5)/ (Ti^(★)/48)/ precipitates precipitates No. Cu + Mn S^(★)(S^(★)/32) (C/12) (μm) (mm⁻²) A21 0.2 0.0057 19.173 1 0.04 4.5 × 10⁶ A220.2 0.0089 11.972 1.01 0.04 5.2 × 10⁶ A23 0.27 0.0096 14.994 0.86 0.036.3 × 10⁶ A24 0.2 0.008 13.535 1.67 0.04 7.3 × 10⁶ A25 0.19 0.01477.0611 1.04 0.04 8.9 × 10⁶ A26 0.59 0.0125 26.566 −1.2 0.25 1.5 × 10⁴A27 0.16 −0.065 −1.398 10.5 0.16 4.3 × 10⁴ S^(★) = S − 0.8 × (Ti − 0.8 ×(48/14) × N) × (32/48) Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 6 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr AI (%) (DBTT-° C.) Remarks A21 222 352 46 2.04 0.39 0 −70 ISA22 288 402 39 1.87 0.32 0 −60 IS A23 338 454 35 1.68 0.29 0 −50 IS A24329 449 34 1.88 0.28 0 −50 IS A25 383 452 35 1.64 0.29 0 −50 IS A26 238342 43 1.21 0.59 1.73 −60 CS A27 302 433 30 1.65 0.48 0 −50 CS * Note:YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 3

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 7 Sample Chemical Components (wt %) No. C Cu S Al N P B Ti OthersA31 0.0005 0.08 0.023 0.035 0.01 0.044 0.0007 0.057 Si: 0.06 A32 0.00160.1 0.025 0.042 0.0132 0.084 0.001 0.072 Si: 0.16 A33 0.0026 0.16 0.0340.041 0.0148 0.121 0.0009 0.09 Si: 0.21 A34 0.0011 0.09 0.025 0.0250.0114 0.044 0.0007 0.065 Si: 0.09 Si: 0.09 Mo: 0.08 A35 0.0005 0.130.023 0.037 0.011 0.046 0.0008 0.06 Cr: 0.22 A36 0.0038 0.09 0.013 0.0320.0012 0.042 0.0005 0 A37 0.0014 0 0.009 0.055 0.012 0.12 0.0005 0.14Si: 0.13

TABLE 8 (Mn/55 + Average size of Number of Sample Cu/63.5)/ (Ti^(★)/48)/(Al/27)/ precipitates precipitates No. S^(★) (S^(★)/32) (C/12) N^(★)(N^(★)/14) (μm) (mm⁻²) A31 0.0072 5.5772 0.99 0.0031 5.78 0.04 3.9 × 10⁶A32 0.0059 8.5273 0.91 0.0034 6.41 0.04 5.5 × 10⁶ A33 0.0077 10.539 0.830.0033 6.4 0.03 6.2 × 10⁶ A34 0.007 6.47 0.85 0.0032 4.01 0.04 5.3 × 10⁶A35 0.0071 9.2382 1.11 0.0034 5.58 0.04 5.9 × 10⁶ A36 0.0148 3.0737 00.0048 3.43 0.25 5.5 × 10⁶ A37 0 0 17.2 0 −1.6 0.16 4.3 × 10⁴ S^(★) = S− 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Ti^(★) = Ti − 0.8 × ((48/14)× N + (48/32) × S) N^(★) = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 9 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr (DBTT-° C.) AI (%) Remarks A1 211 352 44 2.11 0.34 −40 0 IS A2269 408 37 1.98 0.37 −40 0 IS A3 331 452 34 1.81 0.33 −40 0 IS A4 241392 36 1.89 0.41 −50 0 IS A5 224 384 39 1.81 0.37 −40 0 IS A6 233 359 371.11 0.62 −60 1.56 CS A7 283 425 33 1.81 0.57 −40 0 CS * Note: YS =Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 4

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 10 Sample Chemical Components (wt %) No. C Mn Cu S Al N P B TiOthers A1 0.0006 0.11 0.06 0.017 0.05 0.0113 0.042 0.0009 0.055 Si: 0.05A2 0.0012 0.09 0.12 0.027 0.038 0.0141 0.08 0.001 0.077 Si: 0.11 A30.0026 0.1 0.11 0.035 0.024 0.0158 0.12 0.0008 0.096 Si: 0.09 A4 0.00120.08 0.08 0.024 0.049 0.0135 0.032 0.0009 0.073 Si: 0.12 Mo: 0.075 A50.0026 0.11 0.11 0.043 0.046 0.0155 0.03 0.0011 0.104 Si: 0.09 Cr: 0.22A6 0.0034 0.45 0.1 0.0083 0.038 0.0015 0.048 0.005 0 A7 0.0038 0.07 00.012 0.035 0.0024 0.13 0.005 0.17 Si: 0.08

TABLE 11 (Mn/55 + Average size of Number of Sample Cu/63.5)/(Ti^(★)/48)/ (Al/27)/ precipitates precipitates No. Cu + Mn S^(★)(S^(★)/32) (C/12) N^(★) (N^(★)14) (μm) (mm⁻²) A1 0.17 0.0042 22.453 1.50.0032 8.03 0.06 4.4 × 10⁷ A2 0.21 0.007 16.123 1.06 0.004 4.93 0.05 7.0× 10⁷ A3 0.21 0.0069 16.435 1.03 0.0032 3.89 0.06 6.2 × 10⁷ A4 0.160.0048 18.039 1.49 0.0032 7.97 0.06 5.9 × 10⁷ A5 0.22 0.0102 11.7 0.950.0033 7.29 0.06 6.4 × 10⁷ A6 0.55 0.0105 29.751 0 0.0038 5.15 0.25 1.5× 10⁴ A7 0.07 0 −0.542 9.8 0 0 0.04 3.5 × 10⁵ S^(★) = S − 0.8 × (Ti −0.8 × (48/14) × N) × (32/48) Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32)× S) N^(★) = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 12 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr AI(%) (DBTT-° C.) Remarks A1 218 355 44 2.14 0.39 0 −70 IS A2265 402 38 1.85 0.35 0 −60 IS A3 328 455 35 1.68 0.4 0 −60 IS A4 234 36341 2.11 0.37 0 −60 IS A5 219 350 44 2.06 0.35 0 −50 IS A6 202 355 381.59 0.39 0 −60 CS A7 338 458 24 1.31 0.58 0.55 −70 CS * Note: YS =Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 5

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 13 Sample Chemical Components (wt %) No. C Mn P S Al Ti B N OthersA51 0.0009 0.11 0.008 0.022 0.039 0.035 0.0007 0.0008 A52 0.0013 0.080.032 0.031 0.043 0.049 0.0009 0.0021 Si: 0.15 A53 0.0025 0.11 0.0580.043 0.028 0.067 0.0005 0.0019 Si: 0.33 A54 0.0017 0.09 0.082 0.0370.047 0.057 0.0011 0.0023 Si: 0.24 Mo: 0.082 A55 0.0016 0.1 0.118 0.0520.022 0.075 0.0012 0.001 Si: 0.31 Cr: 0.13 A56 0.0035 0.45 0.048 0.0090.033 0 0.005 0.0024 A57 0.0031 0.13 0.118 0.012 0.038 0.15 0 0.0021 Si:0.33

TABLE 14 Average/ size of Number of (Mn/55 + precipi- precipi- SampleCu/63.5)/ (Ti^(★)/48)/ tates tates No. S^(★) (S^(★)/32) (C/12) (μm)(mm⁻²) A51 0.0045 14.211 1.78 0.06 3.3 × 10⁵ A52 0.0079 5.8631 1.16 0.063.6 × 10⁵ A53 0.01 6.3706 1.02 0.05 3.8 × 10⁶ A54 0.01 5.255 0.93 0.053.6 × 10⁶ A55 0.0135 4.3217 1.54 0.05 3.8 × 10⁶ A56 0.0125 20.927 −1.20.26 2.6 × 10³ A57 −0.065 −1.165 10.5 0.06 4.5 × 10⁵ S^(★) = S − 0.8 ×(Ti − 0.8 × (48/14) × N) × (32/48) Ti^(★) = Ti − 0.8 × ((48/14) × N +(48/32) × S)

TABLE 15 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr AI(%) (DBTT-° C.) Remarks A51 189 295 49 2.21 0.35 0 −50 IS A52209 332 45 1.93 0.28 0 −50 IS A53 315 362 41 1.96 0.22 0 −50 IS A54 234380 36 1.75 0.24 0 −40 IS A55 238 407 38 1.63 0.21 0 −50 IS A56 243 33944 1.38 0.42 3.6 −40 CS A57 225 404 38 1.79 0.43 0 −40 CS * Note: YS =Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 6

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 16 Sample Chemical Components (wt %) No. C P S Al Ti B N OthersA61 0.0008 0.008 0.023 0.042 0.059 0.0007 0.0103 A62 0.0017 0.035 0.0250.044 0.074 0.001 0.0135 Si: 0.13 A63 0.0025 0.061 0.034 0.039 0.0950.0009 0.015 Si: 0.24 A64 0.0012 0.085 0.025 0.024 0.066 0.0007 0.0117Si: .11 Mo: 0.06 A65 0.0006 0.12 0.023 0.038 0.061 0.0008 0.0112 Cr:0.13 A66 0.0038 0.042 0.013 0.032 0 0.0005 0.0012 A67 0.0014 0.12 0.0090.055 0.14 0.0005 0.012 Si: 0.13

TABLE 17 Average/size of Number of Sample (Ti^(★)/48)/ (Al/27)/precipitates precipitates No. (C/12) N^(★) (N^(★)/14) (μm) (mm⁻²) A610.67 0.003 7.32 0.05 6.3 × 10⁵ A62 1.03 0.0032 7.06 0.05 6.3 × 10⁵ A631.31 0.0024 8.59 0.05 8.4 × 10⁶ A64 0.81 0.0033 3.77 0.05 7.3 × 10⁶ A651.12 0.0034 5.78 0.05 6.2 × 10⁶ A66 −1.2 0.0048 3.43 0.05 4.5 × 10⁵ A6717.2 −0.018 −1.6 0.28 3.5 × 10³ Ti^(★) = Ti − 0.8 × ((48/14) × N +(48/32) × S) N^(★) = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48),

TABLE 18 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr (DBTT-° C.) AI(%) Remarks A61 209 349 44 2.03 0.25 −60 0 IS A62282 399 37 1.72 0.24 −50 0 IS A63 339 457 34 1.73 0.27 −50 0 IS A64 219360 42 2.21 0.29 −50 0 IS A65 354 449 33 1.73 0.21 −60 0 IS A66 189 34845 1.32 0.43 −40 0.94 CS A67 335 457 26 1.53 0.24 −40 0 CS * Note: YS =Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 7

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 19 Sample Chemical Components (wt %) No. C Si Mn P S Al Ti B NOthers A71 0.0009 0 0.11 0.038 0.017 0.053 0.058 0.0005 0.0119 A720.0012 0.11 0.09 0.053 0.026 0.038 0.076 0.001 0.0147 Si: 0.11 A730.0008 0.1 0.11 0.109 0.033 0.015 0.094 0.0008 0.0158 Si: 0.1 A74 0.00120.12 0.1 0.032 0.024 0.049 0.073 0.0009 0.0133 Si: 0.12 Mo: 0.05 A750.0026 0.09 0.11 0.03 0.043 0.046 0.104 0.0011 0.0155 Si: 0.09 Cr: 0.28A76 0.0018 0 0.68 0.045 0.009 0.048 0.057 0.0004 0.0021 A77 0.0037 0.050.1 0.114 0.01 0.008 0 0.0011 0.0067 Si: 0.05

TABLE 20 (Mn/55 + Average size of Number of Sample Cu/63.5)/(Ti^(★)/48)/ (Al/27)/ precipitates precipitates No. S^(★) (S^(★)/32)(C/12) N^(★) (N^(★)/14) (μm) (mm⁻²) A71 0.0035 18.419 1.38 0.0031 8.790.05 6.3 × 10⁵ A72 0.007 7.512 0.93 0.0042 4.64 0.05 6.3 × 10⁵ A73 0.00610.703 3.46 0.0031 2.5 0.05 8.4 × 10⁶ A74 0.0045 12.864 1.61 0.003 8.510.05 7.3 × 10⁶ A75 0.0102 6.2698 0.95 0.0033 7.29 0.05 6.2 × 10⁶ A76−0.018 −21.59 5.62 −0.009 −2.9 0.05 4.5 × 10⁵ A77 0.0198 2.9383 −2.10.0095 0.44 0.28 3.5 × 10³ S^(★) = S − 0.8 × (Ti − 0.8 × (48/14) × N) ×(32/48) Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32) × S) N^(★) = N − 0.8× (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 21 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%)r_(m) Δr (DBTT-° C.) AI(%) Remarks A71 215 357 46 2.04 0.39 −40 0 IS A72243 382 41 1.89 0.35 −50 0 IS A73 271 425 34 1.75 0.27 −50 0 IS A74 232371 42 1.84 0.24 −50 0 IS A75 226 364 41 1.89 0.22 −60 0 IS A76 189 34742 1.92 0.42 −40 0 CS A77 293 418 36 1.32 0.34 −60 3.51 CS * Note: YS =Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 8

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 22 Sample Chemical components (wt %) No. C P S Al Cu Ti B N OthersB81 0.0021 0.009 0.011 0.037 0.09 0.017 0.0005 0.0011 B82 0.0017 0.0260.01 0.026 0.11 0.021 0.0009 0.0024 B83 0.0018 0.05 0.012 0.027 0.080.015 0.0004 0.0005 Si: 0.02 B84 0.0028 0.082 0.01 0.032 0.12 0.0180.0007 0.0015 Si: 0.18 B85 0.0021 0.113 0.011 0.034 0.12 0.021 0.0010.0018 Si: 0.24 B86 0.0017 0.082 0.008 0.033 0.09 0.017 0.0007 0.0019Si: 0.18 Mo: 0.074 B87 0.0022 0.082 0.01 0.029 0.12 0.019 0.0006 0.0016Si: 0.18 Cr: 0.21 B88 0.0022 0.063 0.008 0.029 0.11 0.055 0.0005 0.0012B89 0.0033 0.12 0.009 0.037 0 0 0.0008 0.0027

TABLE 23 Number of Sample Average size of precipitates No.(Cu/63.5)/(S★/32) Cs precipitates (μm) (mm⁻²) B81 12.8 19.043 0.06 1.8 ×10⁶ B82 24 10.957 0.06 2.1 × 10⁶ B83 8.52 18 0.06 2.5 × 10⁶ B84 23.323.286 0.05 3.2 × 10⁶ B85 24.9 13.843 0.06 4.1 × 10⁶ B86 26.5 11.5290.06 3.2 × 10⁶ B87 27.4 15.471 0.05 4.1 × 10⁶ B88 −2.8 −75 0.08 4.5 ×10⁵ B89 0 33 0.08 6.2 × 10⁴ S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) ×(32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N +(48/32) × S)

TABLE 24 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI(%) (MPa) (DBTT-° C.) Remarks B81 183 305 49 1.930.32 0 37 −40 IS B82 193 332 48 1.88 0.32 0 41 −50 IS B83 204 349 441.88 0.29 0 47 −50 IS B84 267 402 39 1.75 0.27 0 67 −60 IS B85 329 45036 1.65 0.19 0 37 −50 IS B86 325 455 35 1.61 0.31 0 41 −50 IS B87 333449 34 1.66 0.24 0 45 −50 IS B88 232 348 43 1.92 0.29 0 0 −50 CS B89 279453 29 1.22 0.48 3.8 92 −70 CS * Note: YS = Yield strength, TS = TensileStrength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr =In-plane anisotropy index, AI = Aging Index, SWE = Secondary WorkingEmbrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 9

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 25 Sample Chemical Components (wt %) No. C Mn P S Al Cu Ti B NOthers B91 0.0019 0.11 0.008 0.008 0.038 0.12 0.01 0.0008 0.0011 B920.0018 0.14 0.024 0.011 0.042 0.14 0.008 0.0007 0.0015 B93 0.0015 0.090.041 0.009 0.034 0.1 0.009 0.0005 0.0005 Si: 0.08 B94 0.0027 0.1 0.0830.011 0.046 0.11 0.017 0.0008 0.0013 Si: 0.18 B95 0.0022 0.11 0.1 0.0110.039 0.15 0.016 0.0005 0.002 Si: 0.28 B96 0.0019 0.1 0.083 0.010 0.0330.13 0.013 0.0009 0.0021 Si: 0.27 Mo: 0.11 B97 0.0025 0.09 0.076 0.0130.033 0.11 0.02 0.0011 0.0021 Si: 0.31 Cr: 0.24 B98 0.0022 0.47 0.0510.008 0.031 0 0.042 0.0007 0.0016 B99 0.0037 0.13 0.12 0.013 0.034 0.030 0.005 0.0025 Si: 0.32

TABLE 26 (Mn/55 + Average size of Number of Sample Cu + Cu/63.5)/precipitates precipitates No. Mn (S^(★)/32) Cs (μm) (mm⁻²) B91 0.23 29.119 0.05 2.8 × 10⁶ B92 0.28 17 18 0.05 2.5 × 10⁶ B93 0.19 20.8 15 0.052.8 × 10⁶ B94 0.21 29.6 27 0.05 2.9 × 10⁶ B95 0.26 25.9 22 0.05 3.9 ×10⁶ B96 0.23 20.1 19 0.04 2.5 × 10⁶ B97 0.2 19.9 25 0.04 3.9 × 10⁶ B980.47 −23 −39 0.25 1.7 × 10⁴ B99 0.16 5.45 37 0.08 6.3 × 10⁴ S^(★) = S −0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti^(★) × 12/48) ×10000, Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 27 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI(%) (MPa) (DBTT-° C.) Remarks B91 188 309 48 1.910.32 0 43 −50 IS B92 210 331 46 1.88 0.29 0 44 −40 IS B93 225 357 451.85 0.35 0 39 −50 IS B94 292 399 39 1.75 0.32 0 47 −60 IS B95 343 45234 1.61 0.28 0 53 −50 IS B96 333 447 34 1.66 0.28 0 42 −50 IS B97 328452 35 1.65 0.27 0 55 −60 IS B98 201 351 41 1.92 0.45 0 0 −50 CS B99 312437 31 1.21 0.2 4.5 89 −50 CS * Note: YS = Yield strength, TS = TensileStrength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr =In-plane anisotropy index, AI = Aging Index, SWE = Secondary WorkingEmbrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 10

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 28 Sample Chemical Components (wt %) No. C P S Al Cu Ti B N OthersB01 0.0019 0.008 0.008 0.039 0.09 0.006 0.0005 0.0088 B02 0.0017 0.0270.01 0.042 0.14 0.007 0.0005 0.0072 B03 0.0018 0.042 0.009 0.038 0.120.007 0.0007 0.01 Si: 0.07 B04 0.0016 0.086 0.011 0.04 0.1 0.016 0.0010.0125 Si: 0.14 B05 0.0026 0.12 0.018 0.062 0.16 0.045 0.0009 0.0139 Si:0.2 B06 0.0025 0.044 0.025 0.055 0.09 0.065 0.0006 0.012 Si: 0.11 Mo:0.084 B07 0.0022 0.043 0.009 0.033 0.12 0.029 0.0009 0.01 Cr: 0.27 B080.0025 0.041 0.012 0.054 0 0.063 0.0005 0.0012 B09 0.0054 0.11 0.0110.055 0.09 0 0.001 0.011 Si: 0.15

TABLE 29 Average size of Number of Sample (Cu/63.5)/ (Al/27)/precipitates precipitates No. (S^(★)/32) (N^(★)/14) Cs (μm) (mm⁻²) B012.57 2.1 19 0.06 2.8 × 10⁶ B02 4.2 2.6 17 0.06 3.7 × 10⁶ B03 3.04 1.8118 0.06 3.5 × 10⁶ B04 2.43 1.75 16 0.05 4.7 × 10⁶ B05 5.63 3.81 26 0.045.5 × 10⁶ B06 5.75 7.44 25 0.05 4.3 × 10⁶ B07 7.41 2.97 22 0.04 5.2 ×10⁶ B08 0 −2.8 −77 0.2 2.5 × 10⁴ B09 1.67 2.03 54 0.05 4.4 × 10⁶ S^(★) =S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti^(★) × 12/48)× 10000, Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32) × S) N^(★) = N − 0.8× (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 30 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI (MPa) (DBTT-° C.) Remarks B01 209 325 50 1.91 0.350 48 −50 IS B02 219 344 47 1.83 0.29 0 38 −40 IS B03 217 355 43 1.880.31 0 42 −50 IS B04 292 411 36 1.79 0.29 0 43 −50 IS B05 339 450 331.66 0.25 0 55 −40 IS B06 248 390 38 1.75 0.32 0 52 −50 IS B07 243 38939 1.77 0.35 0 45 −40 IS B08 202 339 40 1.99 0.52 0 0 −50 CS B09 291 43132 1.28 0.19 3.9 104 −40 CS * Note: YS = Yield strength, TS = TensileStrength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr =In-plane anisotropy index, AI = Aging Index, SWE = Secondary WorkingEmbrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 11

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 31 Sample Chemical Components (wt %) No. C Mn P S Al Cu Ti B NOthers B11 0.0014 0.1 0.007 0.008 0.042 0.09 0.009 0.0005 0.0094 B120.0016 0.13 0.023 0.011 0.052 0.08 0.01 0.0007 0.0076 B13 0.0017 0.090.044 0.01 0.053 0.08 0.018 0.0009 0.011 Si: 0.07 B14 0.0012 0.1 0.0840.009 0.035 0.11 0.02 0.0008 0.0128 Si: 0.12 B15 0.0024 0.13 0.117 0.0150.061 0.16 0.055 0.0011 0.0142 Si: 0.09 B16 0.0025 0.11 0.035 0.0260.028 0.09 0.038 0.0009 0.013 Si: 0.11 Mo: 0.072 B17 0.0022 0.12 0.0330.009 0.043 0.09 0.04 0.0009 0.014 Si: 0.09 Cr: 0.25 B18 0.0018 0.520.045 0.009 0.035 0 0.06 0.006 0.0022 B19 0.0042 0.11 0.127 0.01 0.0430.09 0 0.005 0.0018 Si: 0.08

TABLE 32 Average size of Number of (Mn/55 + precipi- precipi- SampleCu + Cu/63.5)/ (Al/27)/ tates tates No. Mn (S^(★)/32) (N^(★)/14) Cs (μm)(mm⁻²)) B11 0.19 6.11 2.28 14 0.06 1.1 × 10⁷ B12 0.21 6.91 3.23 16 0.069.5 × 10⁶ B13 0.17 5.62 2.86 17 0.06 1.7 × 10⁷ B14 0.21 6.66 1.7 12 0.051.9 × 10⁷ B15 0.29 24.3 5.68 24 0.05 3.2 × 10⁷ B16 0.2 4.42 1.27 25 0.053.8 × 10⁷ B17 0.21 14.1 3.1 22 0.04 4.5 × 10⁷ B18 0.52 −15 −2 −79 0.251.8 × 10⁴ B19 0.2 8.66 4.85 42 0.06 8.3 × 10⁵ S^(★) = S − 0.8 × (Ti −0.8 × (48/14) × N) × (32/48), Cs = (C − Ti^(★) × 12/48) × 10000, Ti^(★)= Ti − 0.8 × ((48/14) × N + (48/32) × S) N^(★) = N − 0.8 × (Ti − 0.8 ×(48/32) × S)) × (14/48)

TABLE 33 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI (MPa) (DBTT-° C.) Remarks B11 201 321 48 1.94 0.340 35 −40 IS B12 211 342 46 1.89 0.31 0 42 −50 IS B13 221 359 45 1.910.35 0 36 −60 IS B14 269 410 37 1.77 0.32 0 39 −60 IS B15 332 462 331.63 0.31 0 47 −60 IS B16 237 360 42 1.85 0.31 0 53 −60 IS B17 227 35342 1.83 0.33 0 55 −50 IS B18 184 352 39 1.99 0.45 0 0 −50 CS B19 343 45325 1.27 0.21 6.2 93 −60 CS * Note: YS = Yield strength, TS = TensileStrength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr =In-plane anisotropy index, AI = Aging Index, SWE = Secondary WorkingEmbrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 12

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 34 Sample Chemical Components (wt %) No. C Mn P S Al Ti B N OthersB21 0.0018 0.08 0.011 0.008 0.037 0.007 0.0004 0.0014 B22 0.0015 0.050.052 0.009 0.044 0.008 0.0006 0.0016 B23 0.0029 0.11 0.08 0.011 0.0290.02 0.0009 0.0017 B24 0.0025 0.09 0.108 0.011 0.032 0.011 0.0007 0.0027Si: 0.14 B25 0.0017 0.07 0.089 0.015 0.038 0.031 0.0009 0.0042 Mo: 0.077B26 0.0026 0.12 0.093 0.011 0.039 0.014 0.001 0.0031 Cr: 0.14 B27 0.00210.45 0.045 0.009 0.038 0.058 0.0007 0.0021 B28 0.0024 0.32 0.11 0.0080.024 0 0.007 0.0013

TABLE 35 Number Sample Average size of of precipitates No.(Mn/55)/(S★/32) Cs precipitates (μm) (mm⁻²) B21 7.37 18 0.06 1.2 × 10⁵B22 4.11 15 0.06 1.2 × 10⁵ B23 22.7 23.657 0.05 1.8 × 10⁵ B24 5.76 250.05 2.2 × 10⁶ B25 8.83 13.3 0.05 3.1 × 10⁶ B26 8.65 26 0.04 3.7 × 10⁶B27 −14 −72 0.06 3.4 × 10⁴ B28 18.8 24 0.22 2.3 × 10³ S★ = S − 0.8 × (Ti− 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti− 0.8 × ((48/14) × N + (48/32) × S)

TABLE 36 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI(%) (MPa) (DBTT-° C.) Remarks B21 189 301 51 2.020.35 0 43 −50 IS B22 227 356 44 1.97 0.32 0 39 −50 IS B23 259 409 381.81 0.27 0 59 −60 IS B24 321 459 34 1.58 0.21 0 54 −50 IS B25 280 44732 1.59 0.24 0 35 −40 IS B26 313 457 32 1.49 0.21 0 53 −50 IS B27 211354 40 1.96 0.33 0 0 −40 CS B28 254 454 25 1.56 0.28 0 −70 CS * Note: YS= Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 13

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 37 Sample Chemical Components (wt %) No. C P S Al Ti B N OthersB31 0.0011 0.009 0.011 0.039 0.005 0.0006 0.0084 B32 0.0014 0.05 0.0080.053 0.009 0.0008 0.0072 B33 0.0026 0.084 0.013 0.062 0.031 0.00080.0089 Si: 0.11 B34 0.0017 0.11 0.01 0.05 0.051 0.001 0.013 Si: 0.27 B350.0026 0.033 0.012 0.033 0.041 0.0007 0.012 Si: 0.23 Mo: 0.055 B360.0028 0.11 0.011 0.05 0.019 0.0011 0.0095 Si: 0.18 Cr: 0.12 B37 0.00130.055 0.01 0.052 0.052 0.0007 0.0019 B38 0.0038 0.12 0.012 0.022 00.0009 0.003

TABLE 38 Number Sample Average size of of precipitates No.(Al/27)/(N★/14) Cs precipitates (μm) (mm⁻²) B31 1.96 11 0.06 3.5 × 10⁶B32 3.74 14 0.06 3.2 × 10⁶ B33 6.06 26 0.05 4.1 × 10⁶ B34 6.65 17 0.055.3 × 10⁶ B35 2.95 26 0.05 4.4 × 10⁶ B36 3.18 28 0.04 5.9 × 10⁶ B37 −3.6−63 0.21 1.8 × 10⁴ B38 1.79 38 0.07 2.2104 Cs = (C − Ti★ × 12/48) ×10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti −0.8 × (48/32) × S)) × (14/48)

TABLE 39 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI(%) (MPa) (DBTT-° C.) Remarks B31 188 312 51 1.990.31 0 36 −40 IS B32 217 344 45 1.88 0.25 0 37 −50 IS B33 271 404 38 1.70.23 0 52 −50 IS B34 330 458 32 1.74 0.31 0 42 −50 IS B35 220 362 411.89 0.29 0 58 −50 IS B36 333 453 32 1.59 0.21 0 58 −60 IS B37 196 35541 1.32 0.43 0 0 −40 CS B38 329 452 27 1.21 0.18 5.2 88 −40 CS * Note:YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) =Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = AgingIndex, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS =Comparative steel

EXAMPLE 14

First, steel slabs were prepared in accordance with the compositionsshown in the following tables. The steel slabs were reheated and finishhot-rolled to provide hot rolled steel sheets. The hot rolled steelsheets were cooled at a rate of 400° C./min., wound at 650° C.,cold-rolled at a reduction rate of 75%, followed by continuous annealingto produce cold rolled steel sheets. At this time, the finish hotrolling was performed at 910° C., which is above the Ar₃ transformationpoint, and the continuous annealing was performed by heating the hotrolled steel sheets at a rate of 10° C./second to 830° C. for 40 secondsto produce the final cold rolled steel sheets.

TABLE 40 Sample Chemical Components (wt %) No. C Mn P S Al Ti B N OthersB41 0.0015 0.12 0.009 0.007 0.039 0.01 0.0008 0.0073 B42 0.0018 0.080.024 0.009 0.042 0.008 0.0005 0.0094 B43 0.0012 0.09 0.044 0.008 0.0430.009 0.0007 0.0079 Si: 0.06 B44 0.0026 0.11 0.077 0.012 0.054 0.0220.0008 0.011 Si: 0.12 B45 0.0018 0.11 0.11 0.016 0.052 0.051 0.00110.0125 Si: 0.11 B46 0.0021 0.1 0.041 0.013 0.067 0.033 0.0009 0.0083 Si:0.09 Mo: 0.056 B47 0.0019 0.11 0.041 0.008 0.042 0.019 0.0006 0.0095 Cr:0.33 B48 0.0016 0.68 0.045 0.009 0.048 0.052 0.0004 0.0021 B49 0.00370.1 0.114 0.01 0.008 0 0.0011 0.0067 Si: 0.05

TABLE 41 Average size of Number of Sample (Mn/55)/ (Al/27)/ precipitatesprecipitates No. (S^(★)/32) (N^(★)/14) Cs (μm) (mm⁻²) B41 5.66 2.92 150.06 5.1 × 10⁶ B42 2.52 2.17 18 0.06 4.9 × 10⁶ B43 3.55 2.77 12 0.06 5.8× 10⁶ B44 3.91 3.03 26 0.05 6.9 × 10⁶ B45 9.03 5.31 18 0.05 8.1 × 10⁶B46 7.71 8.19 21 0.05 6.8 × 10⁶ B47 5.44 2.98 19 0.04 8.8 × 10⁶ B48 −25−3.3 −62 0.21 1.8 × 10⁴ B49 2.94 0.44 37 0.07 8.3 × 10⁵ S^(★) = S − 0.8× (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti^(★) × 12/48) × 10000,Ti^(★) = Ti − 0.8 × ((48/14) × N + (48/32) × S) N^(★) = N − 0.8 × (Ti −0.8 × (48/32) × S)) × (14/48)

TABLE 42 Mechanical Properties Sample YS TS El BH value SWE No. (MPa)(MPa) (%) r_(m) Δr AI(%) (MPa) (DBTT-° C.) Remarks B41 194 311 49 1.980.41 0 38 −50 IS B42 209 325 47 1.82 0.37 0 45 −40 IS B43 219 355 431.79 0.39 0 38 −50 IS B44 267 395 39 1.71 0.29 0 48 −40 IS B45 322 45933 1.51 0.25 0 39 −60 IS B46 239 360 41 1.61 0.26 0 44 −50 IS B47 233368 42 1.57 0.28 0 41 −50 IS B48 185 348 42 1.92 0.42 0 0 −40 CS B49 378461 27 1.12 0.34 4.1 96 −60 CS * Note: YS = Yield strength, TS = TensileStrength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr =In-plane anisotropy index, AI = Aging Index, SWE = Secondary WorkingEmbrittlement, IS = Inventive Steel, CS = Comparative steel

The preferred embodiments illustrated in the present invention do notserve to limit the present invention, but are set forth for illustrativepurposes. Any embodiment having substantially the same constitution andthe same operational effects thereof as the technical spirit of thepresent invention as defined in the appended claims is encompassedwithin the technical scope of the present invention.

INDUSTRIAL APPLICABILITY

As apparent from the above description, according to the cold rolledsteel sheets of the present invention, the distribution of fineprecipitates in Ti-based IF steels allows the formation of minutecrystal grains, and as a result, the in-plane anisotropy index islowered and the yield strength is enhanced by precipitation enhancement.

1. A cold rolled steel sheet with superior formability, the cold rolled steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities, wherein the composition satisfies the following relationships: 1≦(Cu/63.5)/(S*/32)≦30 and S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48), and wherein the steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.
 2. The cold rolled steel sheet according to claim 1, wherein the composition further comprises 0.01-0.3% of Mn, and satisfies the following relationship: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less.
 3. The cold rolled steel sheet according to claim 1, wherein the N content is 0.004-0.02%, and the composition satisfies the following relationships: 1≦(Al/27)/(N*/14)≦10 and N=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less.
 4. The cold rolled steel sheet according to claim 1, wherein the composition further comprises 0.01-0.3% of Mn, and 0.004-0.02% of N, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10 and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.
 5. A cold rolled steel sheet with superior formability, the cold rolled steel sheet having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, by weight, and the balance of Fe and other unavoidable impurities, wherein the composition satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10, where the N content is 0.004% or more, and S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and wherein the steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.
 6. The cold rolled steel sheet according to claim 1 or 5, wherein the C, Ti, N and S contents satisfy the following relationships: 0.8≦(Ti*/48)/(C/12)≦5.0 and Ti=Ti−0.8×((48/14)×N+(48/32)×S).
 7. The cold rolled steel sheet according to claim 6, wherein the C content is 0.005% or less.
 8. The cold rolled steel sheet according to claim 1 or 5, wherein solute carbon (Cs) [Cs=(C−Ti*×12/48)×10000 in which Ti*=Ti−0.8×((48/14)×N+(48/32)×S), provided that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and Ti contents, is from 5 to
 30. 9. The cold rolled steel sheet according to claim 8, wherein the C content is from 0.001 to 0.01%.
 10. The cold rolled steel sheet according to any one of claims 1 to 5, wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
 11. The cold rolled steel sheet according to any one of claims 1 to 5, wherein the number of the precipitates is 1×10⁶/mm² or more.
 12. The cold rolled steel sheet according to claim 1 or 5, wherein the P content is 0.015% or less.
 13. The cold rolled steel sheet according to claim 1 or 5, wherein the P content is from 0.03% to 0.2%.
 14. The cold rolled steel sheet according to claim 1 or 5, wherein the composition further comprises at least on kind of 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr.
 15. The cold rolled steel sheet according to claim 1 or 5, wherein the composition further comprises 0.01 to 0.2% of Mo.
 16. The cold rolled steel sheet according to claim 14, wherein the composition further comprises 0.01 to 0.2% of Mo.
 17. The cold rolled steel sheet according to any one of claim 2, 4 or 5, wherein the sum of Mn and Cu is from 0.05% to 0.4%.
 18. The cold rolled steel sheet according to any one of claim 2, 4 or 5, wherein the Mn content is from 0.01 to 0.12%.
 19. The cold rolled steel sheet according to claim 2, 4 or 5, wherein the value of (Mn/55+Cu/63.5)/(S*/32) is from 1 to
 9. 20. The cold rolled steel sheet according to claim 3, 4 or 5, wherein the value of (Al/27)/(N*/14) is from 1 to
 6. 21. A method for producing a cold rolled steel sheet with superior formability, the method comprising the steps of: reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities, the composition satisfying the following relationships: 1≦(Cu/63.5)/(S*/32)≦30 and S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48); hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet; cooling the hot rolled steel sheet at a rate of 300° C./min or higher; winding the cooled steel sheet at 700° C. or lower; cold rolling the wound steel sheet; and continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.
 22. The method according to claim 21, wherein the composition further comprises 0.01 to 0.3% of Mn, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less.
 23. The method according to claim 21, wherein the N content is 0.004-0.02%, and the composition satisfies the following relationships: 1≦(Al/27)/(N*/14)≦10 and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less.
 24. The method according to claim 21, wherein the composition further comprises 0.01 to 0.3% of Mn and 0.004 to 0.02% of N, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10 and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.
 25. A method for producing a cold rolled steel sheet with superior formability, the method comprising the steps of: reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, by weight, and the balance of Fe and other unavoidable impurities, the composition satisfying a relationship: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10, where the N content is 0.004% or more, S*=S−0.8×(Ti 0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48); hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet; cooling the hot rolled steel sheet at a rate of 300° C./min or higher; winding the cooled steel sheet at 700° C. or lower; cold rolling the wound steel sheet; and continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.
 26. The method according to claim 21 or 25, wherein the C, Ti, N and S contents satisfy the following relationships: 0.8≦(Ti*/48)/(C/12)≦5.0 and Ti=Ti−0.8×((48/14)×N+(48/32)×S).
 27. The method according to claim 26, wherein the C content is 0.005% or less.
 28. The cold rolled steel sheet according to claim 21 or 25, wherein solute carbon (Cs) [Cs=(C−Ti*×12/48)×10000 in which Ti*=Ti−0.8×((48/14)×N+(48/32)×S), provided that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and Ti contents, is from 5 to
 30. 29. The method according to claim 28, wherein the C content is from 0.001 to 0.01%.
 30. The method according to any one of claims 21 to 25, wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
 31. The method according to any one of claims 21 to 25, wherein the number of the precipitates is 1×10⁶/mm² or more.
 32. The method according to claim 21 or 25, wherein the P content is 0.015% or less.
 33. The method according to claim 21 or 25, wherein the P content is from 0.03% to 0.2%.
 34. The method according to claim 21 or 25, wherein the composition further comprises at least one kind or 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr.
 35. The method according to claim 21 or 25, wherein the composition further comprises 0.01 to 0.2% of Mo.
 36. The method according to claim 34, wherein the composition further comprises 0.01 to 0.2% of Mo.
 37. The method according to claim 22, 24 or 25, wherein the sum of Mn and Cu is from 0.08% to 0.4%.
 38. The method according to claim 22, 24 or 25, wherein the Mn content is from 0.01 to 0.12%.
 39. The method according to claim 22, 24 or 25, wherein the value of (Mn/55+Cu/63.5)/(S*/32) is from 1 to
 9. 40. The method according to claim 23, 24 or 25, wherein the value of (Al/27)/(N*/14) is from 1 to
 6. 